Hello,
Zoology is a very good optional subject because the syllabus is more of static,the questions asked are direct in nature, you can present your answers with diagrams which if done correctly and neatly will fetch you marks, it also overlaps with few part of General Studies Papers, similarly in case of IFos too this optional is available(only difference is eligibility of IFos is Bachelor's Degree in Science/Engineering).
Now, coming to your question, one of the most important thing you have to remember is the syllabus of optional is nothing short of graduation studies, hence the decision to choose zoology or physics depends on you completely, if you choose physics you will have an added advantage is that you're already acquainted with the syllabus, so the efforts you'll put in this will be comparatively less. And most importantly, don't go by the success ratio of any optional subjects, choosing an optional should depend entirely on your interest, the reason I'm emphasizing on this is because many engineering aspirants too have chosen their area of study as optional and have qualified the exam as well. So, choose your options accordingly weighing everything.
Whenever you select an Optional subject, you should check whether it interests you and you can have access to all the material you need. Truth be told, not many Candidates opt for Zoology as their optional subject. In 2010, 421 Candidates opted for Zoology and 23 cleared the exams. That was highest in any year. But the benefit of opting for Zoology is that it is a scoring subject and the syllabus is pretty much well defined. Ecology, Economic Zoology and Evolution topics from Zoology will also help you in your prelims preparation for General Studies. The questions asked are direct and straight forward. The Candidates choosing Zoology are less and so the competition is less as well. So you can opt for Zoology as an Optional subject:)
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